1. Field of the Invention
The present invention relates to an arrayed waveguide grating used, for example, for a wavelength-division multiplexer/demultiplexer in the field of optical wavelength-division multiplex communication or the like.
2. Description of the Related Art
Recently, optical wavelength-division multiplex communication has been practically used, in which a number of optical signals having different wavelengths are multiplexed and transmitted in a single optical fiber. In systems realizing such multiplex communication, an optical multiplexer/demultiplexer for multiplexing/demultiplexing optical signals according to their wavelengths is an important element.
As such an optical multiplexer/demultiplexer, a bulk-type diffraction grating, a dielectric multiplayer element, and the like are known. However, these known devices have problems such as (i) difficulty of determining each selected wavelength, (ii) high manufacturing cost due to complicated manufacturing processes, (iii) large optical loss, and the like; thus, it has been difficult to apply these devices to wavelength-division multiplex communication for multiplexing/demultiplexing a number of different wavelengths.
Recently, the arrayed waveguide grating, disclosed in H. Yamada, et al, xe2x80x9c10 GHz-spacing arrayed-waveguide grating with phase-error-compensating a-Si filmxe2x80x9d, Proceedings of the 1996 Electronics Society Conference of IEICE, C-162, p. 162, 1996, has become the focus of attention. FIG. 7 is a plan showing an example of the arrayed waveguide grating disclosed in the above document.
The shown arrayed waveguide grating consists of one or more input waveguides 71, an input-side slab waveguide 72 connected to the input waveguides 71 for receiving signal(s) from the input waveguides 71, an arrayed waveguide 73 composed of a number of waveguides, connected to the other side of the slab waveguide 72, an output-side slab waveguide 74 connected to the other side of the arrayed waveguide 73, and one or more output waveguides 75 connected to the other side of the slab waveguide 74.
An optical signal incident on the input waveguide 71 is input into the input-side slab waveguide 72, and the optical signal is further input into the arrayed waveguide 73 (composed of a number of waveguides) at the same phase. The input end of the arrayed waveguide 73 and the output end of the input waveguides 71 are respectively arranged to form circles, where the radius of the relevant circle of the arrayed waveguide 73 is twice as much as the radius of the relevant circle of the input waveguides 71, and the positional relationship is such that the center of the relevant circle of the arrayed waveguide 73 corresponds to a position on the relevant circle of the input waveguides 71.
In the arrayed waveguide 73, each waveguide is adjusted so as to provide the same phase difference between any two adjacent waveguides, and the output-side slab waveguide 74 is connected to the other end of the arrayed waveguide, as explained above. Similar to the input side, in the arrangement of the arrayed waveguide 73, the output-side slab waveguide 74, and the output waveguides 75, the output end of the arrayed waveguide 73 and the input end of the output waveguides 75 are respectively arranged to form circles, where the radius of the relevant circle of the arrayed waveguide 73 is twice as much as the radius of the relevant circle of the output waveguides 75, and the positional relationship is such that the center of the relevant circle of the arrayed waveguide 73 corresponds to a position on the relevant circle of the output waveguides 75.
According to the above structure, a wavelength-division multiplexed optical signal incident on the input waveguide 71 is divided into signals having different wavelengths, and each signal is output from an output waveguide 75 corresponding to the relevant wavelength, thereby realizing the wavelength multiplexing/demultiplexing function.
Generally, in optical wavelength division multiplexing (WDM) communication, the ON/OFF state, light intensity, wavelength, or the like of the optical signal should be monitored for each wavelength at some points in the transmission path. In order to execute such a monitoring operation, the optical signal is divided into signals having different wavelengths by using an arrayed waveguide or the like, and the optical signal of a target wavelength is further divided into a main optical signal and a monitored optical signal by using an (optical) fiber coupler or the like, and then the monitored optical signal is monitored using a photodetector or the like.
However, in the above monitoring method, the same number of fiber couplers as the number of the wavelength channels are necessary, and thus the system is complicated and the system cost is increased due to the necessary cost and space for providing the fiber couplers. Additionally, in this case, after the division of an optical signal using a wavelength multiplexer/demultiplexer, the optical power of the main optical signal is again decreased by further dividing the signal using fiber couplers or the like. Therefore, the power loss of the main optical signal is large.
In order to solve the above problem, Japanese Unexamined Patent Application, First Publication, No. Hei 10-303815 discloses an optical wavelength-division multiplexer/demultiplexer having a monitor function explained later in detail, in which a function of monitoring the optical signal of each wavelength is added to the wavelength-division multiplexing/demultiplexing function necessary for the WDM communication, thereby omitting fiber couplers or the like, and decreasing the cost, size, and optical loss of the WDM communication system.
FIG. 8 is a plan showing the optical wavelength-division multiplexer/demultiplexer having the monitor function disclosed in the above publication. The disclosed system comprises an input waveguide 81, an input-side slab waveguide 82 connected to the input waveguide 81 for receiving a signal from the input waveguide 81, an arrayed waveguide 83 composed of a number of waveguides, connected to the other side of the slab waveguide 82, an output-side slab waveguide 84 connected to the other side of the arrayed waveguide 83, N output waveguides 85 connected to the other side of the slab waveguide 84, and N monitoring waveguides 86 used for the monitoring operation. The optical signal is wavelength-divided by the arrayed waveguide, and transmitted through the output-side slab waveguide 84 and converged onto the output waveguides 85. Simultaneously, the optical signal is wavelength-divided by the arrayed waveguide due to interference of the next order to the main order of interference (i.e., order of diffraction) of the arrayed waveguide, and the N monitoring waveguides 86 are positioned where these wavelength-divided optical signals (related to the next order of interference) converge.
Here, the difference xcex94xcex8 between diffracted optical signals (i.e., diffracted light beams) having a difference xe2x80x9cixe2x80x9d of the diffraction order (i.e., order of diffraction) therebetween can be defined as follows while xcex94xcex8 is sufficiently small:
xcex94xcex8=ixcex/(nd)xe2x80x83xe2x80x83(1)
where n indicates the effective refractive index in the output-side slab waveguide, xcex indicates the wavelength of the optical signal, and d indicates the pitch of the arrayed waveguide at the joint (portion) of the arrayed waveguide and the output-side slab waveguide.
In FIG. 8, if it is assumed that (i) xe2x80x9caxe2x80x9d is the center point of the arrayed waveguide 83 at the joint of the arrayed waveguide 83 and the output-side slab waveguide 84, (ii) xe2x80x9cbxe2x80x9d is an end of the output waveguide 85, at which an optical signal of a target wavelength is converged via the output-side slab waveguide 84, and (iii) xe2x80x9ccxe2x80x9d is an end of the monitoring waveguide 86, at which the corresponding monitored optical signal is converged via the output-side slab waveguide 84, then the relevant monitoring waveguide 86 is arranged in a manner such that the angle between the line a-b and the line a-c is xcex94xcex8 indicated in the above formula (1).
An optical signal having another wavelength is converged on another output waveguide 85; thus, the monitoring waveguides 86 corresponding to each wavelength are respectively arranged in a manner such that each monitoring waveguide 86 is positioned along the direction rotated from the direction of the corresponding output waveguide 85 (related to the target wavelength) by angle xcex94xcex8 in the formula (1). Generally, the diffraction order m is determined so as to provide the maximum diffraction efficiency of the main optical signal; thus, the most efficient diffraction order of the diffracted optical signal used for monitoring is m+1 or mxe2x88x921, and in this case, the optical signal can be most efficiently monitored.
Under the above conditions, the angle xcex94xcex8 between the output waveguide 85 (for the main optical signal) and the corresponding monitoring waveguide 86 is defined as follows:
xcex94xcex8=xcex/(nd)xe2x80x83xe2x80x83(2)
The diffraction efficiency P0 of the diffracted (main) optical signal converged on the output waveguide 85 and the diffraction efficiency P1 of the diffracted (and monitored) optical signal converged on the monitoring waveguide are defined as follows:
P0=1/Sxe2x80x83xe2x80x83(3)
P1=exp (xe2x88x922(xcfx80w/d)2)/Sxe2x80x83xe2x80x83(4)
S=1+2xcexa3[exp(xe2x88x922(i xcfx80w/d)2)]xe2x80x83xe2x80x83(5)
where w indicates the spot size (the 1/e2 half power width of the power distribution of the optical signal parallel to the substrate and perpendicular to the waveguide) of the arrayed waveguide 83 at the joint of the arrayed waveguide 83 and the output-side slab waveguide 84, and xcexa3 indicates the sum from i=1 to ∞.
FIG. 9 shows variations in (i) the diffraction efficiency P0 of the main optical signal, (ii) the diffraction efficiency P1 of the monitored optical signal, and (iii) the ratio P0/P1 when the value w/d calculated using formulas (3) to (5) varies. As obvious from FIG. 9, an arbitrary ratio of distribution (i.e., division) into the main optical signal and the monitored optical signal can be selected by suitably adjusting the pitch d and spot size w of the arrayed waveguide 83 at the joint of the arrayed waveguide 83 and the output-side slab waveguide 84.
However, the pitch d and the width of the waveguide of the arrayed waveguide 83 are fixed to some degree at the joint with the output-side slab waveguide 84; therefore, in order to change the ratio of distribution into the main optical signal and the monitored optical signal, it is necessary to determine the pitch d again.
In consideration of the above circumstances, an objective of the present invention is to provide an arrayed waveguide grating for easily changing the ratio of distribution of an optical signal into the main optical signal and the monitored optical signal.
Therefore, the present invention provides an arrayed waveguide grating comprising:
a substrate;
one or more input waveguides, provided on the substrate;
one or more output waveguides provided on the substrate;
an arrayed waveguide including two or more waveguides arranged in a manner such that the lengths of the arranged waveguides gradually increase by a predetermined length difference;
an input-side slab waveguide for connecting the input waveguides and the arrayed waveguide; and
an output-side slab waveguide for connecting the output waveguides and the arrayed waveguide, and wherein:
an optical signal incident on the input waveguide is input into the arrayed waveguide via the input-side slab waveguide and is divided into main optical signals having different wavelengths in the arrayed waveguide with respect to a diffraction order m, m being a natural number, and the divided optical signals are transmitted through the output-side slab waveguide and converged on the output waveguides, and
the arrayed waveguide grating further comprising:
one or more monitoring waveguides for monitoring optical signals having corresponding wavelengths of the main optical signals, where the monitored optical signals are diffracted in the arrayed waveguide with respect to a diffraction order m+i or mxe2x88x92i, i being a natural number, and the monitored optical signals converge on the monitoring waveguides, and wherein:
the arrayed waveguide has a taper structure in the vicinity of the joint of the arrayed waveguide and the output-side slab waveguide, and in the taper structure, the width of each waveguide gradually changes along the direction of light transmission so as to adjust the ratio of distribution of each optical signal into the main optical signal and the monitored optical signal.
The present invention also provides an arrayed waveguide grating comprising a similar structure, where photodetectors are provided in place of the monitoring waveguides.
Typically, in the taper structure of the arrayed waveguide, the width of each waveguide gradually decreases or increases towards the joint of the arrayed waveguide and the output-side slab waveguide.
According to the present invention, the spot size of the arrayed waveguide can be adjusted by suitably providing the taper structure, and a desired ratio of distribution of each optical signal into the main optical signal and the monitored optical signal can be easily selected without changing the pitch of the arrayed waveguide, or the width of the whole portion of each waveguide of the arrayed waveguide.
When the above photodetectors are used, the photodetectors can be placed in a space provided in the arrayed waveguide grating; thus, the device size including the monitoring system can be smaller.